Wide frequency spectrum television tuner with single local oscillator

ABSTRACT

A tuner includes a conversion (or heterodyning) stage, including a local oscillator and a mixer, for converting an RF signal corresponding to a selected channel to an IF signal. The local oscillator has only one tunable oscillating portion for generating a signal, and a frequency converter, such as a controllable frequency divider, for converting the frequency of the signal generated by said oscillating portion, so that the receiver is able to tune through the UHF and VHF bands. The operation of the requency converter is responsive to a digital controller, such as a microprocessor.

FIELD OF THE INVENTION

The present invention concerns a television receiver having a conversiontuner for producing an intermediate frequency signal from an RF sourcesignal.

BACKGROUND

It is a common practice to use more than one local oscillator to tunethe full frequency range of television RF sources, which for atelevision receiver in the United States, can extend from 55 MHz ofchannel 2 of the lower VHF band to 847 MHz of channel 69 of the UHFband, a frequency range of more than 17:1. This requires that the localoscillators for conventional all band TV tuners cover a wide frequencyrange of from approximately 90 MHz up to almost 1 GHz.

It is desirable that a local oscillator not be required to tune overmore than a range of 2:1, with an outside limit of about 3:1, due toproblems of assuring that the oscillator will oscillate, reliability offrequency and signal amplitude of the generated oscillator signal overthe wide spectrum, and tracking problems, since other circuits, e.g.,tunable filters, must track as the local oscillator frequency ischanged. Further, varactor diodes, which typically are used to tune thelocal oscillator, have a limited range of adjustment and becomenon-linear in the low capacitance part of their adjustment range. Forthese reasons, in a television receiver, at a minimum, two localoscillators are often used with each covering only part of the tunablespectrum, e.g., one for the VHF band and the other for the UHF band.

It is further desirable that a single oscillator be used in order toeliminate beat frequency problems between oscillators, which requiresthat oscillators not in use be turned off. Still further, a reducednumber of oscillators would result in reduced components, reducedprinted circuit space, and reduced cost.

A known apparatus which reduces the local oscillator tuning range isdescribed by H. Fuchs in DE 38 30 921 entitled ARRANGEMENT FOR ADAPTINGA RECEIVER TO DIFFERENT CHANNEL RASTERS. Fuchs describes apparatus forgenerating a conversion signal for a mixer which includes a variableoscillator for generating a first RF signal over a predeterminedfrequency range, a divider for converting the frequency of the first RFsignal in accordance with a binary number, N, to provide a second RFsignal having a frequency range which varies as a function of the binarynumber, N, for application to the mixer; and a microprocessor forselecting values of the binary number, N, to provide a respectivefrequency range for the second RF signal for each selected value of thebinary number, N, for the purpose of reducing memory requirements in amulti-stantard television receiver.

In certain applications, such as applications requiring quadraturedemodulation, it would be desirable to provide a quadrature phaseshifted output signal for a second mixer. An example of a digitalquadrature phase shift generator is described by Huber et al. in U.S.Pat. No. 5121057 entitled MEDIA FLAW DETECTION APPARATUS FOR A MAGNETICDISC DRIVE WITH SQUARING AND SUMMING OF IN-PHASE AND QUADRATURE-PHASEDETECTED SIGNALS. The Huber et al. apparatus includes a divide-by- fourcircuit for generating in-phase and quadrature signals for applicationto respective mixers.

SUMMARY OF THE INVENTION

It is herein recognized that a problem exists in mixer signal generationin that where mixer signal generation involves digital phase shifting,that local oscillator frequency range requirements may require operationof the local oscillator in a frequency range that is greater than thatrequired for conversion in the mixer.

It is an object of the present invention to provide an apparatus and amethod for generating a conversion signal for a mixer which minimizesthe highest frequency range necessary for a local oscillator to cover toprovide the mixer signal.

Apparatus, in accordance with the invention applies to mixer signalgenerators of a type including a variable oscillator means forgenerating a first RF signal over a predetermined frequency range; afrequency conversion means for converting the frequency of the first RFsignal in accordance with a binary number, N, to provide a second RFsignal having a frequency range which varies as a function of the binarynumber, N, for application to the mixer; and a control means forselecting values of the binary number, N, to provide a respectivefrequency range for the second RF signal for each selected value of thebinary number, N.

Apparatus embodying the invention is characterized by first means forapplying digital phase shift to the second RF signal to produce adigitally phase shifted signal; second means for applying analog phaseshift to the second RF signal to produce an analog phase shifted signal;and third means for selecting the analog phase shifted signal forapplication to the mixer as the second RF signal at frequencies above agiven, frequency and for selecting the digitally phase shifted signalfor application to the mixer as the second RF signal at frequenciesbelow the given frequency.

In accordance with a feature of the invention, the control means selectsa value of unity, N=1, for the binary number; and the third meansselects the analog phase shifted signal for application to the mixerwhereby the second RF signal supplied to the mixer occupies a frequencyrange equal to that of the first RF signal produced by the variableoscillator means.

In accordance with a further feature of the invention, the first meanscomprises means for applying true and complemented versions of the firstRF signal to respective inputs of a first divider and a second divider;and means for applying an output of the first divider to a control inputof the second divider for producing at an output of the second divider aphase shifted second RF signal for application to the mixer having afrequency equal to one-half of that of the first RF signal produced bythe variable oscillator means.

A method, in accordance with the invention, applies to mixer signalgeneration of a type including the steps of: generating a first RFsignal over a predetermined frequency range; converting the frequency ofthe first RF signal in accordance with a binary number, N, to provide asecond RF signal having a frequency range which varies as a function ofthe binary number, N, for application to the mixer; and selecting valuesof the binary number, N, to provide a respective frequency range for thesecond RF signal for each selected value of the binary number, N.

The method, in accordance with the present invention is characterized bythe further steps of: applying digital phase shift to the second RFsignal to produce a digitally phase shifted signal; applying analogphase shift to said second RF signal to produce an analog phase shiftedsignal; and selecting the analog phase shifted signal for application tothe mixer as the second RF signal at frequencies above a given frequencyand for selecting the digitally phase shifted signal for application tothe mixer as the second RF signal at frequencies below the givenfrequency.

In accordance with a feature of the invention, the method is furthercharacterized by selecting a value of unity, N=1, for the binary number;and concurrently selecting the analog phase shifted signal forapplication to the mixer whereby the second RF signal supplied to themixer occupies a frequency range equal to that of the first RF signal.

In accordance with yet another feature of the method of the invention,the method step of applying digital phase shift to the second RF signalcomprises: applying true and complemented versions of the first RFsignal to respective inputs of a first divider and a second divider; andapplying an output of the first divider to a control input of the seconddivider for producing at an output of the second divider the analogphase shifted second RF signal for application to the mixer, the analogphase shifted second RF signal having a frequency equal to one-half ofthat of the first RF signal produced by the variable oscillator means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in block diagram form, a prior art television tunerwherein a phase locked loop (PLL) is used to control a voltagecontrolled oscillator (VCO) serving as a first local oscillator (LO) incombination with a mixer for converting an RF input signal to an IFsignal.

FIG. 2 shows, in block diagram form, a VCO according to aspects of thepresent invention, including an oscillatory portion and a count downdivider in a cascade arrangement.

FIG. 3 shows, in block diagram form according to aspects of the presentinvention, an alternate embodiment of the local oscillator of FIG. 2,which is suitable for use in a direct conversion tuner.

FIG. 4 shows wave shapes found in the oscillator of FIG. 3.

FIG. 5 shows, in block diagram form according to aspects of the presentinvention, another alternate embodiment of the oscillator of FIG. 2,which is suitable for use in a direct conversion tuner.

FIG. 6 shows, in block diagram form, a direct conversion tuner suitablefor using the embodiments shown in FIGS. 3 and 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A tuner 1 selects the RF signal corresponding to a selected channel fromthe plurality of RF signals provided by an RF source 3 and converts theselected RF signal to a corresponding IF signal in the conventional IFfrequency range (e.g., in the United States between 41 and 46 MHz). RFsource 3 may, for example, comprise a television broadcast receivingantenna, a cable distribution network, a video cassette recorder, avideo disc player, or a home computer. The IF signal produced by tuner 1is demodulated and separated into luminance, synchronization,chrominance and audio components by a video and audio signal processingunit 5. The various components are further processed by video and audiosignal processing unit 5 to produce the video and audio responsescorresponding to the selected channel.

The present tuner includes an RF input terminal 7 to which the RF source3 is connected. The RF signals received at terminal 7 are filtered by anRF filter section 9 to select the RF signal corresponding to theselected channel and the resultant RF signal is coupled to a first mixer11. A local oscillator (LO) signal having a nominal frequencycorresponding to the selected channel is generated by an LO 13 and alsocoupled to first mixer 11. As shown in FIG. 1, LO 13 comprises a VHFlocal oscillator 13a and a VHF local oscillator 13b, with theappropriate oscillator being chosen by the bandswitching signal fromchannel selector 43, depending upon which channel is selected. Mixer 11heterodynes the selected RF signal and the LO signal to produce an IFsignal. Mixer 11 produces the sum and difference frequency products ofthe LO signal and the selected RF signal. The IF signal is coupled to anIF filter stage 15 which has a passband response selected to pass thedifference frequency product of the LO signal and the selected RFsignal. The frequencies of the LO signal determine the frequency rangeof the IF signal.

LO 13 is a voltage controlled oscillator and RF filter 9 are controlledin response to the magnitude of a tuning control voltage Vt. Themagnitude of the tuning control voltage Vt is controlled in accordancewith the selected channel by a tuner control section 25 including aphase locked loop (PLL). In the PLL, the frequency of the output signalof a crystal oscillator 27 is divided by a frequency divider (÷R) 29 toproduce a frequency reference signal. The PLL also includes a frequencydivider (÷K) 31 and a frequency divider (÷N) 33 for dividing thefrequency of the local oscillator signal to produce a frequency dividedversion of the LO signal. The frequency reference signal and thefrequency divider local oscillator signal are compared by a phasecomparator 35 to produce a pulse signal which represent the magnitudeand sense of frequency deviation between them. The pulse error signal isfiltered by an integrator 37, which serves as a low pass filter, toproduce the tuning control voltage Vt for LO 13. The magnitude of thetuning control voltage is changed until the frequency of the frequencyreference signal and the frequency divided LO signal are substantiallyequal.

Division factor R of divider 29 is selected to determine the frequency(FREF) of the frequency reference signal. Division factor K of divider31 is selected to reduce the frequency of the relatively high frequencyLO signal before further processing and determines together withdivision factor R the division factor K. Division factor N of divider 33is controlled by a control unit 39 to set the frequency of the first LOsignal in accordance with the binary representation of the channelnumber of the desired channel stored in a register 41. The binaryrepresentation of the channel number is entered into channel numberregister 41 in response to a user's operation of a channel selector 43,which may, for example, comprise a calculator-like keyboard by which theuser may enter in sequence the tens and units digits of the channelnumber of the desired channel.

One aspect of the present invention, shown in FIG. 2, presents anembodiment of a local oscillator, e.g., local oscillator 13, which isable to tune over the wide frequency range of frequencies necessary fora television receiver, as discussed above in the background section.

Tuning voltage Vt (see FIG. 1) is applied through an appropriateresistor 60 to varactor diode 62 for varying the capacitance of varactordiode 62 for tuning oscillating portion 64 of VCO 13, which in theexemplary embodiment, is an oscillator which can be tuned from 0.5 to 1GHz. The output signal 66 from oscillator 64 is coupled to a frequencydivider control 68, which is a programmable binary counter which dividesthe oscillatory signal of oscillator 64 by a binary number in responseto a divider control signal applied to terminal 70. The divider controlsignal at terminal 70 can be provided by tuning control microprocessor72 or it can be derived from channel selector 43 of FIG. 1 (not shown).

For a division number of 1, the output signal Vo is the same frequencyas the frequency of oscillator 64, i.e., 500 to 1000 MHz tuned by Vt,and the frequency divider can be considered as a divider bypass. For theoscillating frequency range of 500 MHz to 1000 MHz, as the divisionnumber of divider control 68 increases, the frequency of signal Vodecreases, as follows:

    ______________________________________                                        Division Number of                                                            divider 68                Frequency of Vo                                     ______________________________________                                        N = 2             F = 250 MHz to 500 MHz                                            4                                     125 MHz to 250 MHz                      8                                     62.5 MHz to 128 MHz                     16                                 31.25 MHz to 62.5                    ______________________________________                                                          MHz                                                     

The value of N is chosen for groups of channels and is changed when thechannel chosen is in a different group. This table is an illustration asto the relationship of the frequency of the oscillating portion 64 tothe frequency of output signal Vo with a change of division number fordivider 68. The frequency and range of oscillation of the oscillatingportion 64 can be adjusted as required. Additionally, the frequencieschosen for Vo depend upon whether the sum or difference signals from themixer are used as the IF signal.

In the alternative, oscillating portion 64 can oscillate at a much lowerfrequency, e.g., at a VHF frequency, and control 68 can be a frequencymultiplier (not shown), e.g., a doubler or a tripler. In which case, Vocan achieve frequencies higher than the frequency signal generated byoscillatory portion 64.

Another embodiment is shown in FIG. 3. The embodiment of FIG. 3 has twoadditional divide by 2 circuits 74a, 74b and 90 degree phase shiftedclocks 76 to obtain two output signals of quadrature phase Vo1, Vo2which are useful for direct conversion tuners discussed in connectionwith FIG. 6 below. The wave forms of the embodiment of FIG. 3 are shownin FIG. 4 and show how the 90° degree phase shifted clocks 76 areobtained, using Q and not-Q clocks and dividing them both by two. Theconnection from the output of divider 74a synchronizes the two dividers74a, 74b such that the clock of 74a always leads. This embodiment showsan additional division by dividers 74a, 74b and thus, the voltagecontrolled oscillator 64 is tunable between the higher frequencies of 1GHz to 2 GHz.

Another alternate embodiment with respect to FIG. 3, of a VCO 13 for adirect conversion tuner discussed in connection with FIG. 6 below, isshown in FIG. 5 wherein the quadrature phase signals Vo1, Vo2 for theUHF band is accomplished by two discrete LC phase shifter networks 76a,76b for output frequencies at Vo1, Vo2 above 500 MHz. For localoscillator frequencies below 500 MHz, the arrangement is switched sothat the quadrature phase signals Vo1, Vo2 are produced by the divide by2 divider as previously described with reference to FIG. 3. The outputsignals Vo1, Vo2 are switched between the external components and theextra dividers 74a and 74b by switch 78.

FIG. 6 shows a direct conversion tuner according to PCT patentapplication No. 94/00138. Basically, the direct conversion tunercontains two channels, each with two conversion stages. The received RFsignal is coupled to each of two mixers M1A and M1B via a tuned RFamplifier which provides gain and some selectivity. Desirably, the gainof the RF amplifier is automatically controlled in response to anautomatic gain control (AGC) signal (not shown). The first localoscillator signal generated by a first local oscillator L01, such asshown in FIGS. 3 and 5 discussed above, is tuned to the center frequencywO of the frequency band of the desired channel between the lower sidedband (LSB) and the upper side band (USB). The first local oscillatorsignal is provided in quadrature components, Vo1, Vo2, that are used todrive mixers M1A and M1B. The respective IF output signals of mixers M1Aand M1B are filtered by two low pass filters LPF A and LPF B. Low passfilters LPF A and LPF B provide the necessary selectivity to reject theresponses from the adjacent channels and higher order products of mixersM1A and M1B.

Each of the output signals of mixers M1A and M1B includes both a lowerside band portion and an upper side band portion corresponding to theLSB and USB portions of the received RF signal. The output signal of lowpass filters LPF A and LPF A are coupled to respective ones of a secondpair of mixers M2A and M2B. Mixers M2A and M2B are driven by respectiveones of a second pair of quadrature local oscillator signals generatedby a second local oscillator LO2, such as shown in FIGS. 3 and 5. Eachof the second local oscillator signals has a frequency wN located abovethe cutoff frequency of the low pass filters LPF A and LPF B filters tofulfill the Nyquist criteria. The output signals of mixers M2A and M2Bare added in a summer unit SU to produce an output signal. This outputsignal is coupled to a demodulator (not shown) for demodulation, and thedemodulated resultant is coupled to further signal processing sections.

What is claimed is:
 1. Apparatus for generating a conversion signal fora mixer, comprising:variable oscillator means for generating a first RFsignal over a predetermined frequency range; frequency conversion meansfor converting the frequency of said first RF signal in accordance witha binary number, N, to provide a second RF signal having a frequencyrange which varies as a function of said binary number, N, forapplication to said mixer; and control means for selecting values ofsaid binary number, N, to provide a respective frequency range for saidsecond RF signal for each selected value of said binary number, N; firstmeans for applying digital phase shift to said second RF signal toproduce a digitally phase shifted signal; second means for applyinganalog phase shift to said second RF signal to produce an analog phaseshifted signal; and third means for selecting said analog phase shiftedsignal for application to said mixer as said second RF signal atfrequencies above a given frequency and for selecting said digitallyphase shifted signal for application to said mixer as said second RFsignal at frequencies below said given frequency.
 2. Apparatus asrecited in claim 1 wherein for operation within a highest frequencyrange of said apparatus: said control means selects a value of unity,N=1, for said binary number; andsaid third means selects said analogphase shifted signal for application to said mixer whereby said secondRF signal supplied to said mixer occupies a frequency range equal tothat of said first RF signal produced by said variable oscillator means.3. Apparatus as recited in claim 1 wherein said first meanscomprises:means for applying true and complemented versions of saidfirst RF signal to respective inputs of a first divider and a seconddivider; and means for applying an output of said first divider tocontrol input of said second divider for producing at an output of saidsecond divider a phase shifted second RF signal for application to saidmixer having a frequency equal to one-half of that of said first RFsignal produced by said variable oscillator means.
 4. A method forgenerating a conversion signal for a mixer, comprising:generating afirst RF signal over a predetermined frequency range; converting thefrequency of said first RF signal in accordance with a binary number, N,to provide a second RF signal having a frequency range which varies as afunction of said binary number, N, for application to said mixer;selecting values of said binary number, N, to provide a respectivefrequency range for said second RF signal for each selected value ofsaid binary number, N; applying digital phase shift to said second RFsignal to produce a digitally phase shifted signal; applying analogphase shift to said second RF signal to produce an analog phase shiftedsignal; and selecting said analog phase shifted signal for applicationto said mixer as said second RF signal at frequencies above a givenfrequency and for selecting said digitally phase shifted signal forapplication to said mixer as said second RF signal at frequencies belowsaid given frequency.
 5. A method as recited in claim 4 furthercomprising:selecting a value of unity, N=I, for said binary number; andconcurrently selecting said analog phase shifted signal for applicationto said mixer (MI) whereby said second RF signal supplied to said mixeroccupies a frequency range equal to that of said first RF signal.
 6. Amethod as recited in claim 4 wherein the step of applying digital phaseshift to said second RF signal comprises:applying true and complementedversions of said first RF signal to respective inputs of a first dividerand a second divider; and applying an output of said first divider to acontrol input of said second divider for producing at an output of saidsecond divider, said analog phase shifted second RF signal forapplication to said mixer, said analog phase shifted second RF signalhaving a frequency equal to one-half of that of said first RF signalproduced by said variable oscillator means.